GPS (global positioning system) signals are not particularly good at penetrating dense materials such as walls and windows. This is primarily because GPS signals, at 1575.42 MHz and 1227.60 MHz, have low skin depth when hitting building surfaces. As a result, signals from a GPS satellite can be completely blocked by buildings on the ground.
Because in general a GPS receiver needs signals from at least four satellites to determine its location accurately, when ground buildings block signals from too many GPS satellites, the receiver may suffer significantly in its location accuracy. Further, buildings may reflect GPS signals, so that even if signals of four or more GPS satellites are received, the location accuracy can be reduced.
Still further, even when line-of-sight with four satellites is available, in certain scenarios such as when the GPS receiver is between tall buildings, the satellites having line-of-sight with the receiver may be on closely-parallel paths (e.g., the geometry is such that the angles are all close to straight down), causing what is known as a high dilution-of-precision (DOP). This also reduces accuracy.
Such circumstances are common in downtown areas where streets are surrounded by high rises. This is sometimes referred to as the “GPS urban canyons” problem. Tall trees sometimes can cause similar effects.